(1) Field of the Invention
The present invention relates generally to instruments and more particularly to a transducer that can be used with a strain sensor to make an accelerometer.
(2) Description of the Prior Art
Traditional electrical accelerometers use a magnetic mass supported by a spring in a housing. An induction coil is wound around the interior of the housing. Acceleration is sensed by measuring the electromotive force induced in the induction coil by movement of the magnetic mass. Piezoceramic transducers use a piezoelectric effect to produce a low voltage output in response to force. These sensors usually require a preamplifier to be placed in close proximity. In remote applications, especially those involving arrays of accelerometers, the preamplifiers and telemetry combining signals for transmission to the receive site, can be large and a major factor in the system expense.
Fiber optic sensor systems remove the requirement for preamplification and electronic telemetry. Very low sensitivity accelerometers are based on the acceleration causing two optical waveguides to be misaligned, thus varying the intensity of the light signal. Other accelerometers use large coils of tens of meters of optical fiber. The large amount of fiber and the limited bend diameter of the fiber limit these to fairly large sizes.
Another type of optical accelerometer uses a fiber optic segment positioned between a base and a ridged reaction mass. Acceleration causes the reaction mass to pinch the fiber optic segment against the base. Acceleration can be measured by measuring the modulation of the light passing through the fiber optic segment.
Sometimes it is more convenient to measure strain, and a conventional technique is the use of a force transducer to translate acceleration into strain. Strain sensors include a foil strain gauge having a plurality of foil traces on a backing material. This gauge is mounted on an object to measure elongation of the object. Strain is measured by measuring the increased resistance in the strain gauge. Another linear electrical strain sensor can be constructed by providing mercury or some other conductive material in an elastomeric tube. The elastomeric tube is mounted to the object. Stretching of the tube results in a narrowing of the cross-sectional area which increases electrical resistance in the conductive material. This resistance can be measured giving an indication of strain.
A fiber optic sensor can also be used to measure strain. A piece of fiber optic is provided that has a Bragg grating written in the fiber optic at either end of a sensing region. The sensing region is mounted to an object being measured, and elongation of the sensor can be measured by providing light pulses at a known interval in the fiber optic and collecting the reflected pulses. Elongation of the fiber optic sensor region causes a measurable delay in the pulses.
Navy Case No. 97966, which is incorporated by reference herein, teaches use of a fiber optic laser to measure strain. FIG. 1, below, has additional details concerning the use of fiber laser sensors.
As discussed herein, there is provided an accelerometer having a transducer that can be used with a variety of strain sensors.